Eldrad is not dead, his soul is currently struggling for control of the Blackstone Fortress with it's daemonic center. He still has active spirit stones, and thus can be interred in Wraithguard/Wraithlord. Eventually he'll win the battle of wills to control blackstone and be reclaimed by Ulthwés infinity circuit.
Eldar don't die, they turn into circuit boards!
As a by-the-way, no-one we have ever heard of has faced Eldrad in a direct confrontation. This means that if anyone has managed it, they've lost. Any speculation about "Oh, he can see the future but can't fight worth a damn" has absolutely zero
basis in fact. It's like saying "Oh, that nuke can't do very much, it's just a boring grey tube".
"Also yea he started the war of Armaggedon but is it really that hard to trick a ork into attacking a world ?"
Name anyone else who has ever managed it?
"Also how do Magnus and Ahirman not have control, is it just because they fell to chaos."
Well Magnus couldn't see his own future well enough to change it (he failed to stop the Horus Heresy, and became a demon prince, and was unable to save Prospero) and the only major spell Ahriman cast went catastrophically wrong, with totally unintended effects. Sounds a bit poor to me.